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PROFESSOR: Hi.
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In this clip we're going
to demonstrate the gene
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regulation of the E.
coli lac operon.
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As you remember from the lecture
clips, the lac operon
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is a collection of genes that
produces the lactose
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metabolism machinery
when expressed.
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Now, when these are expressed,
E. coli is able
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to metabolize lactose.
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However, it prefers to
metabolize glucose, to use
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that as its sugar.
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Thus, when there's glucose
available to the cell, it's
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pointless for it to create this
machinery, because it's
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not going to actually use it.
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It's going to metabolize
the glucose instead.
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But when we remove glucose and
add lactose, the E. coli
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absolutely needs to be able to
metabolize this lactose,
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otherwise it's not going to
be able to grow with the
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available energy source.
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Thus, we have a couple of
different situations.
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When we have glucose present
but not lactose, we do not
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want to express the
lac operon.
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However, when we remove glucose
and we add lactose, we
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do want to express
the lac operon.
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Now we've covered when the E.
coli would like to express
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this operon.
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However, how can it express this
operon only when it wants
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to and not when it doesn't?
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That's where the promoter
comes into play.
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The promoter is where RNA
polymerase binds.
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And RNA polymerase needs to bind
to this promoter in order
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to transcribe and express
these genes.
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In this particular promoter,
we have a site where a
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repressor can bind and a site
where an activator can bind.
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So I have a repressor and
I have an activator.
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So when the repressor is bound,
polymerase cannot bind
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to the promoter.
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Polymerase also needs the
activator to be bound to the
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promoter in order to bind.
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So how can the E. coli regulate
when the repressor
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and activator are bound
to the promoter?
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Well, when we have lactose in
the system, I have lactose
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right here, the repressor
is able to bind lactose.
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The repressor only binds
to the promoter
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when there is no lactose.
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And when binding to lactose,
the repressor comes undone
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from the promoter.
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How does the cell tell whether
there's glucose
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in the cell or not?
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When glucose is low in the
cell, the cell makes a
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molecule called cyclic
AMP, or CAMP.
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I have a molecule of
CAMP right here.
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And so CAMP is the cell's way
of indicating that there is
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low glucose in the cell.
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The activator needs
CAMP to bind to be
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activator binding site.
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When CAMP is removed, or
glucose is added, the
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activator cannot bind to
the promoter anymore.
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Ok, so let's give it a try.
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In this first situation
we have glucose,
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we don't have lactose.
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So I have neither my lactose
nor my CAMP in the system.
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Repressor, can you bind?
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The Repressor is bound.
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Activator, can you bind?
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No, the activator
is not bound.
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The repressor is bound, and so
when RNA polymerase comes to
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the promoter, RNA polymerase
is not able to
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bind at this promoter.
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And thus, it's not able to
express these genes.
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And so, as we can see here,
there is no expression.
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In this second situation,
we have no glucose
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and we do have lactose.
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Since I'm removing glucose,
I am adding back CAMP.
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Activator, can you bind?
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Activator is now bound.
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And I'm also adding lactose
to the system.
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Repressor can't bind anymore.
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So what do I have at
my promoter now?
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RNA polymerase comes along
and finds only an
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activator at the promoter.
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The polymerase can thus bind
to the promoter and move
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downstream to transcribe and
express the lactose metabolism
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machinery that we have here.
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So we have now covered how the
E. coli organism is able to
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regulate the transcription
of the
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lactose metabolism machinery.
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This is only one example of many
genes and many organisms
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that are regulated by mechanisms
such as this.
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Gene regulation is a very
widespread mechanism, in order
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to keep a cell from producing
proteins that
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are a waste to produce.
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Thank you for watching.
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